Black liquor chemical recovery unit utilizing gas recirculation



Feb. 23, 1965 F. w. HOCHMUTH 3,170,442

BLACK LIQUOR CHEMICAL RECOVERY UNIT UTILIZING GAS RECIRCULATION Filed Dec. 28, 1962 2 Sheets-Sheet 1 INVENTOR. FRANK W. HOCHMUTH I ther use.

BLACK LIQUOR CHi lMfCAL RECOVERY UNIT UTILIZING GAS RECIRCULATION Frank W. Hochmuth, West Simsbury, Conn, assignor to Combustion Engineering, Inc., Windsor, Comm, a corporation of Delaware Filed Dec. 28, 1962, Ser. No. 248,080

2 Claims. (Cl. 122-7) This invention relates to chemical recovery boilers, and in particular to the construction thereof which will permit the attainment of higher superheat temperatures.

In the sodium base process of paper pulp manufacture, wood chips are cooked in digesters, and the resulting pulp is taken from the digesters, washed and prepared for fur- The so-called black liquor separated from the removed pulp is thereafter passed through evaporators for greater concentration and is then burned in a special furnace to oxidize the organic matter, and to recover the The heat liberated from the burn:

heating steam.

Problems are encountered in the boiler section of these units because of the large amount of chemical ash particles which is contained in the hot combustion gases exhausted from the furnace. When the temperature of the chemical ash particles carried by the combustion gases is above the softening or melting temperature, these sticky particles tend to collect on the steam generating and superheating tubes in such a manner that they are rather difficult-to thereafter remove. For this reason, from the standpoint of clean boiler operation, it is desirable to maintain the temperature of the combustion gases and the ash particles carried thereby below the softening or melting point of the ash before these gases encounter any steam generating surface. In recent years, in order to increase the over-allefficiency of these chemical recovery boilers, a higher superheat temperatures are being asked for. It is not uncommon for a chemical recovery. boilertoday to produce superheated steam of 900 F; and above. I In order to obtain these higher. temperatures, it obviously is ice ' outlet pipe 26. Contained within the evaporator is a rotating drum 23 driven by means of motor 24. Thisv drum in rotating picks up black liquor from the pool contained in the lower portion of the housing, thus exposing it to the hot combustion gases flowing through the upper portion of the housing.

The hot combustion gases generated by the burning of black liquor within the furnace pass into the upper portion of the furnace, flowing first over steam generating tubes 32. Also positioned within the upper portion of the furnace above tubes 32 is final superheater'section 34,

and primary superheater tubes 36, 38 and 40.

The combustion gases, after passing over these'heating surfaces, leave the upper portion of the furnace through opening 42. The tubes extending through the opening 42 are offset with respect to one another so as to allow free passage of gases therethrough. Below theopening 42, the tubes 44 are formed into a single plane, and are finned as shown in FIG. .2, so as to form a solid baflle between the upper furnace and the open gas pass 50. After flowing through this open gas pass 50, the gases make a reverse turn at the bottom thereof, and enter the upwardly extending steam generating portion of the gas pass through opening 52. An ash hopper 53 is positioned beneath the opening 52, so as to permit removal of any ash particles that might settle out at this point as the gases make a turn in direction. I

The second portion of the gas pass contains extensions of steam generating tubes 32. Pinned tubes 54 form an imperforate bathe betweenthe first and second portions ing 52are offset and contain no fins, just as the tubes in a the opening 42 are constructed. Also, some of the tubes 54 arebentout of the panel and form a part of the tube necessary to also increase the temperature of the combustiongases in the superheater. New metals which are corrosion resistant and capable of withstanding exposure to high temperatures are available which make possible the, higher operating temperatures of the chemical recovery superheater.

gases becomes molten or semi-molten in form. These sticky ash particles when contacting heat exchange surface adhere thereto, causing slagging and plugging of these Plugging in thesteam generatiilg'secfion 0f the boiler bee of which is connected to the stack 82 downstream of fan cause of higher combustion gas temperature.

Additionalobjects and advantages of the invention will appear from the following description of a preferred emrated to the desired density, is introduced into the furnace by nozzles 18. The air for combustion is supplied by However, when the temperature of the Y combusion gases leaving 'thesuperheater becomes exces-' sive, the chemical ash particles carried by the combustion means of duct 16 and air nozzles 14. The black liquor 7 screen extending into the upper portion of the furnace. The gases leaving the steam generating section of thegas pass flow through opening 56 into the last portion, which contains theeconomizen tubes 64. A baffle 58 separates these twoportions of the'gas pass. I

p Also positioned between these two passages are down comers 60 which extend through battle plates 62. Since The combustion gases flow by means of duct '65 into the evaporator 20, and are then exhausted to the atmospherethroughstack 821 Fan maintainsa steadylfiow of these gases to the atmosphere. Duct 84, the inlet end 80, and the outlet end ofwhich is connected to nozzles 86, can be used for recirculating gases back to theopengas pass '50 when desired. Thermocouple 92 is positioned within the open pass 50, and measures the temperature of the combustion gases atthis point. Signals from thermo; couple 92 are received by control 94, which regulates the positions of both sets of dampers 96 and 98. In this mannerthe temperature. of the combustion gases leaving the open gas pass 50 is maintained below a predetermined maximum.

Water enters the economizer tubes 64 by means of inlet header 66, and flows by way of outlet header 68 into the steam and water drum 7t). Downcomers on supply Water Patented Feb. 23, 1965 a} I from-header 76 into tubes 78 which line all four of the walls of the furnace 10. a

' Asseen in FIG. 2, tangent tube '78 and finnedtubes 9t) the turnace 10, the volume and temperatures of the com- .bu'stion gases thereby created will be greater'than during the periods when only a small amount of blackliquor is being burned; For this reason, the amountv of recirculated gas introduced into the open pass 50 will vary with the load of the unit. In other Words, when black liquor is being burned at maximum capacity, or at full load, a large amount of cool gas is recirculated in order to prevent the temperature of the gases leaving open pass 50 from exceeding a predetermined maximum. The recirculated gas is at a temperature substantially below that of the gases fiowing from the su'perheater section, since it has given upheat in the steam generating section, the economizer, and the evaporator. These recirculated gase s will be 1000 to 1200 F. cooler than the gases coming from the superheater zone. -As less black liquor is burned, the need for cooled recirculated gases also becomes less. Thus it can be seen that as the temperature sensed by and alsoyca n be easily removed therefrom.

difiicult to thereafter remove by means of sootblowing or other conventional .ash deposit removal equipment, whereas if the dust particles are in the solid state they do not attach themselves to the heat exchanger as readily While the preferred embodiment of the invention has been shown and described, it will be understood ithat minor changes in construction may be made .without de-- parting from the spirit and scope of'the invention as claimed. I v a WhatIclairnisr 9' 1. In a system for recovering'chemicals and generating and superheating steam from the combustion of blackliq-s nor, the combination of a smelter furnace defined by gen- :erally vertical walls, means for supplying black liquor into the furnace interior and for burning the same in the lower portion thereof for producing combustion gases laden j with chemical ash particles, a superheater heat exchanger positioned within the upper portion of the furnace a gas. pass connected to the upper portion of the furnace, said a gas pass comprised of walls forming a tortuous gas passage Y madeup of a plurality; of upwardly and downwardly ex thermocouple 92 increases, control 94 will move the dampers 98 in'a closing direction, while simultaneously movingdampers 06 in an opening direction. Conversely,

asthe temperature sensed by thermocouple 92 decreases,

dampers 98 are moved in an opening directionwhile dampers 96am simultaneously movedina closing direction. The hot combustiongases laden with chemical dust particles, leaving the upper portion of the furnace, will Heat is also the wall's" of-this pass 50. When theunit is operating at maximum capacity, the temperature of the gases flowing enter open gas pass 50 by means of opening 42. During their downward flow: through the open gas pass 50, the dust particles, which are generally 100 F. or more above 0 a the temperature of theconibu'stion gases, cool down by giving up heat to the combustion gases. f transferred from the combustion gases to the tube lining through the openpass is also substantially reduced by.

discharging a large amount of relatively cool recirculated gases through nozzles 86. During periods when 3 less black liquor is burned, and thus the temperatures sensed by thermocouple 92 decreases, less gas will be re- "circulated. i V

From the above it'will be seen that higher combustion gas temperatures can be allowed'in the superheatei' sections, while still preventing excessive combustion'gas temperatures and/or semi-moltenchemicaldust particles I from entering the steam generating section of the boiler;

. For-exampla by utilizing gas recirculation introduced into ,open gas pass 50, it might be possible to allowthe hot combustion gases to leave the superheater section and enter the opengas pass 50 at a temperature of 1 500 F. or. higher when firingthe unit attmaximum capacity;

The ltemperaturie' of the chemical dust particles carried therebywill then be somewhat higher. In'passingthrough the open. gas pass .50, the temperature of both the combustion gases and the'dust' particles will be reduced by Q means of the recirculated gas, andv also because ofthe heattransfer-torthe tubes lining the walls ofthis ,pass.

:3 The combustion gasesentering the steam generating section of thegas pass should be reduced in'temperature to approximately 1100" F. in order to insure the solidifi:

cation of the dustp'articles prior to their contact with'the' particles will become molten or'serni-molten and sticky steam generating tubes; Above this temperature, the f tending "interconnected passages, the walls. of the gas: passage being lined with fluid cooled tubes, a first portion of the gas passage being connected to the upper portion of the-furnace,'-said first portion being an open gas pass free of any heat exchange inembers other than the fluid cooled tubes lining the walls thereof,a secondportion of the gas passage being downstream of the .first portion 7 passage at th e other end, a fan-positioned within thestack, firstvdamper means. positioned within'the stack down i p stream of the fan, a duct having'an inletend and anoutf j let end,the inlet end being'connected'to the stackiup stream of the first damper means but downstream of the fan, with the outlet end ofthe duct connected to the 'first portion of'the gas passage, second damper means posi- I I tionedin said duct, temperature sensing' rneans positioned 'in saidfirst portion ofthe gas passage, saidlternperature sensing mean's being operatively connected tothe first and seconddamper means. so as to .movethe first damper means in a closing direction 'whenan increase in temperature is sensed while moving the second damper means in an opening direction, and moving the first damper means in an opening direction when a decrease in temperature is sensed while moving the second damper means'in a clos ing directionpto thereby allow the attainment of lhigher superheat temperature while still preventing Iundue slag-I ging of the steam generating tubes positioned within the second portion of the gasp'assageQ V p a 2. The 'systern set forth in claim L'including a black liquor evaporator positioned intermediatethe-Stack; and the second ortionoflthe gaspassage, throughlwhich the combustion gases pass. I a

, ReterencesCitedinthe file of this'patentl UNITED ISTATES PATENTS Q OTHER-REFERENCES V 'ing,1955 (page20-slO reliedon),;'. V i

'Steam, Its Generation and'Use', published. by th'e 'Babcock andiWilc'oxCo; New York 27th. ed, 3rdfprint- 

1. IN A SYSTEM FOR RECOVERING CHEMICALS AND GENERATING AND SUPERHEATING STEAM FROM THE COMBUSTION OF BLACK LIQUOR, THE COMBINATION OF A SMELTER FURNACE DEFINED BY GENERALLY VERTICAL WALLS, MEANS FOR SUPLYING BLACK LIQUOR INTO THE FURNACE INTERIOR AND FOR BURNING THE SAME IN THE LOWER PORTION THEREOF FOR PRODUCING COMBUSTION GASES LADEN WITH CHEMICAL ASH PARTICLES, A SUPER HEATER HEAT EXCHANGER POSITIONED WITHIN THE UPPER PORTION OF THE FURNACE, A GAS PASS CONNECTED TO THE UPPER PORTION OF THE FURNACE, SAID GAS PASS COMPRISED OF WALLS FORMING A TORTUOUS GAS PASSAGE MADE UP OF A PLURALITY OF UPWARDLY AND DOWNWARDLY EXTENDING INTERCONNECTED PASSAGES, THE WALLS OF THE GAS PASSAGE BEING LINED WITH FLUID COOLED TUBES, A FIRST PORTION OF THE GAS PASSAGE BEING CONNECTED TO THE UPPER PORTION OF THE FURNACE, SAID FIRST PORTION BEING AN OPEN GAS PASS FREE OF ANY HEAT EXCHANGE MEMBERS OTHER THAN THE FLUID COOLED TUBES LINING THE WALLS THEREOF, A SECOND PORTION OF THE GAS PASSAGE BEING DOWNSTREAM OF THE FIRST PORTION AND IN SERIES FLOW RELATIONSHIP THEREWITH, A PLURALITY OF STEAM GENERATING TUBES POSITIONED WITHIN THE SECOND PORTION OF THE GAS PASSAGE, A STACK OPEN TO ATMOSPHERE AT ONE END AND CONNECTED TO THE SECOND PORTION OF THE GAS PASSAGE AT THE OTHER END, A FAN POSITIONED WITHIN THE STACK, FIRST DAMPER MEANS POSITIONED WITHIN THE STACK DOWNSTREAM OF THE FAN, A DUCT HAVING AN INLET END AND AN OUTLET END, THE INLET END BEING CONNECTED TO THE STACK UPSTREAM OF THE FIRST DAMPER MEANS BUT DOWNSTREAM OF THE FAN, WITH THE OUTLET END OF THE DUCT CONNECTED TO THE FIRST PORTION OF THE GAS PASSAGE, SECOND DAMPER MEANS POSITIONED IN SAID DUCT, TEMPERATURE SENSING MEANS POSITIONED IN SAID FIRST PORTION OF THE GAS PASSAGE, SAID TEMPERATURE SENSING MEANS BEING OPERATIVELY CONNECTED TO THE FIRST AND SECOND DAMPER MEANS SO AS TO MOVE THE FIRST DAMPER MEANS IN A CLOSING DIRECTION WHEN AN INCREASE IN TEMPERATURE IS SENSED WHILE MOVING THE SECOND DAMPER MEANS IN AN OPENING DIRECTION, AND MOVING THE FIRST DAMPER MEANS IN AN OPENING DIRECTIONWHEN A DECREASE INTEMPERATURE IS SENSED WHILE MOVING THE SECOND DAMPER MEANS IN A CLOSING DIRECTION, TO THEREBY ALLOW THE ATTAINEMNT OF HIGHER SUPERHEAT TEMPERATURE WHILE STILL PREVENTING UNDUE SLAGGING OF THE STEAM GENERATING TUBES POSITIONED WITHIN THE SECOND PORTION OF THE GAS PASSAGE. 